AAAAI: Oldest Siblings Hit Hardest by Allergies

Action Points

Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

Explain that a Japanese survey study found no protective effect for different types of allergies for first-born children; in fact, food allergies appeared more common among firstborns.

Note that the study was observational and was based on parent answers to a questionnaire.

SAN FRANCISCO -- Firstborn children may be at greater risk of food allergies and hay fever than subsequent siblings, but higher birth order doesn't affect the risk of asthma, Japanese researchers found.

The picture was less clear-cut for risk of eczema, with birth order playing no role at school ages but having a significant impact in infancy (P=0.32 and P=0.003 for trend, respectively), Takashi Kusunoki, MD, of Kyoto University in Kyoto, Japan, and colleagues reported here at the American Academy of Allergy, Asthma & Immunology meeting.

Although the observational study could not draw causal links, these findings may help in counseling parents of children with allergies about the risk if they choose to have more children, commented session moderator Melinda Rathkopf, MD, an allergist in private practice in Anchorage, Alaska.

"They always want to know: 'Do you think my next child will have allergies?'" she told MedPage Today.

Previous studies have helped in that regard but typically lumped all allergic diseases together, she noted.

Kusunoki's group surveyed parents of more than 13,000 school children ages 7 to 15 in Kyoto about specific symptoms indicative of allergic diseases and analyzed the relationship to birth order in the 11,454 children for whom this factor was known.

After adjustment for age, sex, gestational age and birthweight, family history of allergy, day care attendance before age 1 year, and other factors, the researchers found no variation in bronchial asthma prevalence between firstborns, secondborns, and later birth order (P=0.91 for trend).

But a clear downward trend in prevalence at school age appeared for the following:

Allergic rhinitis at 30% among firstborns, 26% among secondborns, and 21% among third- and later-born children (P<0.0001 for trend)

In infancy, eczema declined in prevalence for second- and later-born children compared with those who were their parents' first (P=0.003 for trend). The same was true for food allergy in infancy (P<0.0001 for trend).

Wheezing in infancy, though, was more common with more older siblings (P<0.0001 for trend), which Kusunoki chalked up to the effect of more respiratory infections brought home by older children.

A second survey including blood sampling for mite and pollen allergens on nearly all 11-year-olds at primary schools in the Shiga Prefecture suburbs around Kyoto (618 students in total) showed that higher birth order protected against sensitization to one or more allergens (P=0.02 for trend).

This was the case for cedar and cypress allergen sensitization (P=0.01 and P=0.03 for trend, respectively) but not mite or orchard grass (both P=NS for trend).

One popular explanation is the hygiene hypothesis, under which the immune system is suspected to benefit from older kids bringing home respiratory infections and endotoxins to expose younger siblings at at early age, Rathkopf noted.

However, "the fact that the effect was also observed for eczema and food allergy in infancy suggests a prenatal origin," Kusunoki told attendees at the session.

Prior pregnancies may change a woman's immunological milieu in ways that affect subsequent children, he suggested.

Both agreed that whatever the mechanism, the results likely would generalize to other countries despite Japan's generally lower birth rate and smaller family size.

Kusunoki noted that while allergic disease was parent-reported rather than physician-diagnosed, the questionnaire was specific -- for example, in the case of food allergy, it asked about hives, cough, and other immediate reactions after eating certain foods.

The researchers reported having no conflicts of interest to disclose.

Rathkopf reported having no relevant conflicts of interest to disclose.

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.